In Malcolm Gladwell's book, The Tipping Point, he describes the conditions that are necessary to transform Hush Puppies from the old school to new cool. "The world of the tipping point is a place where the unexpected becomes expected, where radical change is more than a possibility," he argues. "It is - contrary to all our expectations - a certainty."

American public opinion appears to be approaching just such a point in relation to the war in Iraq. The last fortnight has revealed a growing impatience with the military misadventure in the Gulf and an irritation with the White House's persistent denials that anything is wrong. This has translated into more urgent and widespread calls to bring the troops home that has finally percolated up to the political class. This new phase has put George Bush on the back foot, forcing him to deliver a major address tomorrow night to rally public support, which is evidently draining away. He will tell them that America needs "resolve". For the White House Iraq has become the latest faith-based initiative.

A recent Gallup poll revealed that 56% said the war "wasn't worth it". Meanwhile, for the first time, a majority say they would be "upset" if Bush sent more troops, and a new low of 36% say troop levels should be maintained or increased. An earlier Washington Post poll showed that two-thirds of the public believe the US military is bogged down in Iraq while almost three- quarters think the level of casualties is unacceptable. The figures match or exceed the previous high-water mark of public disenchantment. More than half believe the war has not made them safer and 40% believe it has striking similarities to the experience in Vietnam.

Anti-war sentiment had always been part of mainstream national conversation here. But with the Democratic party and its presidential candidate having supported the war, such views remained marginal in the body politic. Now, as these statistics make themselves felt in the postbags and phone logs of congressmen, the notion that not only is the war not going to plan but that the plan might itself be flawed is finding expression in the most unlikely places. On June 16, the Republican congressman Walter Jones, the man largely responsible for introducing freedom fries to the congressional menu, co-sponsored a bipartisan resolution persuading the president to set a timetable for troop withdrawal.

When the secretary of defence, Donald Rumsfeld, testified before a Senate armed services committee last week, the Republican senator from South Carolina, Lindsey Graham, said: "I'm here to tell you sir, in the most patriotic state that I can imagine, people are beginning to question. And I don't think it's a blip on the radar screen. I think we have a chronic problem on our hands. We will lose this war if we leave too soon. And what is likely to make us do that? The public going south. And that is happening."

The critical factor driving this slump, explains Christopher Gelpi, associate professor of political science at Duke University who specialises in public attitudes to foreign policy, is not how many soldiers they lose but whether the mission for which they have fallen is likely to be successful. "The most important single fact is that the public perceive the mission as being destined for success. The American public is partly casualty-phobic but it is primarily defeat phobic. You can muster support for just about any military operation in the US so long as you can get enough of the defeat-phobic people on board."

Those who are casualty-phobic have been troubled by the 1,739 slain soldiers. So far this month, the US has, on average, had almost three soldiers killed and 10 wounded, every day. The 700 Iraqis who have died in the last month do not figure on the sympathy radar. But the chaos of which their deaths are just the most bloody indicator suggests little likelihood of success.

Comparisons with Vietnam are premature, but the trend towards it in public perception is undeniable. "It won't be easy, but they could carry on at this level of support for quite some time. But if it drops another 10%, that would be really bad," says Gelpi. The decisive moment that produced the tipping point in Vietnam was the Tet offensive; given the ideological incoherence and fractured organisation of the Iraqi insurgency the turning point is likely to be less dramatic and more prolonged. It may even have happened already.

Until earlier this year, the White House had an easy-to-follow narrative for success on its own terms. When weapons of mass destruction were not found, it simply changed the story to fit the absence of facts. The final chapter then became the democratisation of the Arab world. First there would be a "handover" of power, then elections, all leading up to Iraqis regaining control of their own country. The carnage, in terms of human life, regional stability and international law, was dismissed as a price worth paying for the bigger picture. For a while, a majority of the American public bought it. But in recent months they have proved reluctant to wear it.

You can keep spinning just so long before you fall flat on your face. The administration's insistence that things are on track and all it must do is stay the course is beginning to grate. US efforts to reshape the world through a policy of pre-emption have been buttressed by an attempt to remould reality through the power of assertion. Since Vice-President Dick Cheney claimed that the insurgency was "in its last throes" 77 American soldiers and about 600 Iraqi civilians have died. His tortured explanation, late last week, that "if you look at what the dictionary says about throes, it can still be a violent period", adds insult to injury.

"We are all capable of believing things which we know to be untrue," wrote George Orwell in his essay In Front of Your Nose. "And then, when we are finally proved wrong, impudently twisting the facts so as to show that we were right. Intellectually, it is possible to carry on this process for an indefinite time: the only check on it is that sooner or later a false belief bumps up against solid reality, usually on a battlefield."